The present invention relates in general to instruments for measuring the vapor deposition quantities and rates of vacuum coating apparatus and, in particular a new and useful oscillator crystal measuring head and arrangement.
Quartz oscillators have long been employed in thin layer deposition operations, for determining the layer thickness and for measuring the rate of coating (i.e. the mass of substance deposited per unit time), during thin layer deposition operation. An example is in vacuum evaporation techniques. In such a process, a layer is deposited on an oscillator crystal of a quartz controlled oscillator which is accommodated in the evaporator, and the mass of the layer changes the frequency of the oscillator crystal. The determined change in frequency may then serve as a measure of the thickness of the deposited layer and the frequency change per unit time as a measure of the rate of coating. (See for example, German OS 2 053 154, U.S. Pat. Nos. 3,383,842 and 3,541,894, French Pat. Nos. 1,389,513).
A prerequisite for employing this known oscillator crystal measuring method, particularly if optical thin layer systems are deposited, is a sensitive and reliably operating apparatus for measuring the frequency of the crystal. In prior art devices, the frequency of the measuring oscillator, depending on the mass deposited on the crystal, has been compared with a high-stability reference oscillator. Recently, however, the frequency of the crystal is often measured directly, by a digital frequency counter. The rate of coating can be determined from the frequency variation per unit time.
Experience has shown that one of the most important sources of trouble and error during operation of such devices is in the mounting of the oscillator crystal. This becomes manifest already if the crystal is simply removed from its holder without any other change, and then reinserted into place. The frequency is changed. Even abrupt frequency jumps may occur during an operation, which probably are caused by mechanical or thermal strain on the retained crystal. Such trouble appears particularly with conventional oscillator crystal holders in which the rim of a circular quartz slab is pressed, by springs, against an annular face, and disturbances may be due to an uneven contact face or to dust particles which penetrated between the contact face and the crystal during the clamping operation.
To overcome these difficulties, it has already been provided to hold the oscillator crystal fast only at two points of its circumference. For this purpose, two parts of fixing jaws are used, for example, between which the quartz slab is clamped. It is a disadvantage however, that this makes the frequency required exchange of the crystals complicated. That is, the fixing jaws must be spread apart to be able to remove one quartz slab and reinsert another. Skill is needed for the operation and the loss of time connected thereto is undesirable in an industrial application of thin layers deposition techniques.
To obtain a mounting which satisfies the requirements of measurement and permits an easy exchange of the crystals, a solution has been proposed which is illustrated in FIG. 1 of the accompanying drawing. This design will now be briefly explained to permit ease in understanding the present invention.
FIG. 1 shows a housing of an oscillator crystal measuring head comprising two parts 1 and 2. Part 2 is provided with a window 3 and, with the measuring head employed in a coating apparatus (with vapor deposition or cathode sputtering), the window will be oriented to face the stream of particles to be deposited. This stream is directed from mostly below, upwardly, so that the measuring head is used in the shown position (with its longitudinal axis extending vertically).
Along the periphery of window 3, part 2 is provided with at least two contact faces 4 for the oscillator crystal slab 6, which faces have the configuration of circular segments. Two contact pins 11, 12 are supported by the upper part 1 of the measuring head, in a supporting plate 13 of this part and by means of insulating guide bushings 14, 15. A spring 16 presses pins 11, 12 against the contact faces, so that crystal slab 6 is firmly held in its position. Spring 16 in turn is supported on a column 17 which is carried by, and insulated against, supporting plate 13 and provided with a connecting lug 18 for an electrical conductor 19 leading to the outside through a bushing insulator 20 passed through a closing plate 21 of part 1 of the measuring head. It is frequently advisable however, to accommodate the oscillator crystal along with the other parts of the measuring oscillator within the same housing. Then, the housing provides an electrical shielding at the same time. In such a case, bushing 20 may be used not only for leading the measurement signal away but also for supplying operating voltage for the oscillator.
The crystal has two metallic coatings in a known manner. The two metallic coatings of the oscillator crystal slab are electrically contacted on the underside, by the two metallic contact faces 4 which are in electrical connection with the grounded housing, and on the top side by the two contact pins 11 and 12. It is a common practice to provide only the central portion of each of the two sides of the oscillator crystal slab with a metallic coating, with metallic conducting tracks leading to the periphery, for example, semi-circular contact faces. To ensure a correct position of the upper and lower housing parts upon assembly, a fixing or snap-action element may be provided, such as a pin 22 which permits closing of the housing only in a definite position of the two parts.
The arrangement described in the foregoing makes it possible to remove the oscillator crystal slab from the mount and insert it agian, without causing an appreciable frequency change. Disturbances due to inattentive insertion, dust particles, compression, etc., have not been observed either. A disadvantage remains however, in that no longer continuous operation is possible, because of the necessary frequent exchange of the crystals.